1. Field of the Invention
The invention generally relates to the field of geophysical subsurface seismic imaging in the field of geophysical seismic exploration. More specifically, this invention generally relates to machines, program products, and methods to generate filtered seismic images based on seismic image data filtered by attenuating coherent noise from unfiltered seismic image data using a plurality of nonstationary convolution operators as local filters at each spatial location of an unfiltered seismic image wavefield.
2. Description of Related Art
In the oil and gas industry, geophysical seismic data processing and analysis techniques are commonly used to aid in the search for, and evaluation of, subterranean hydrocarbon deposits. In many instances, a seismic energy source can be used to generate seismic energy signals that propagate into the earth and are, at least partially, reflected by subsurface seismic reflectors such as interfaces between underground formations having different acoustic impedances. Such seismic energy reflections can subsequently be recorded in a geophysical time series by seismic energy detectors, sensors, or receivers positioned at a recording surface located at or near the surface of the earth, in a body of water, or at known depths in boreholes. The resulting seismic data then can be processed and analyzed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
Processed seismic data, in the form of surface waves delineating a wavefield, can be useful in delineating and mapping the earth's subsurface features. Geophysical time series data acquired in the field, however, often contains undesired dispersive energy, such as coherent seismic noise, as well as desired seismic energy. Such unwanted coherent seismic noise can interfere with, overwhelm, or otherwise mask desired seismic energy useful for subsurface explorations. Particularly, when propagating through an irregular surface or complex near surface, such as, rugged topography or complex geology, surface waves can have complicated wave paths and lateral velocity variations resulting in unwanted dispersive energy. Much of this energy propagates horizontally in the near surface of a wavefield. In the presence of complex geology or rugged topography, this energy can have a complicated wave path, thus making its removal a challenging task. Nevertheless, removing this dispersive energy is an important and beneficial step that is necessary, in many cases, to focus the subsurface seismic image so that it can be used for accurate and cost-effective subsurface exploration.
Current solutions attenuate unwanted dispersive energy and coherent seismic noise from seismic image data by downwardly continuing the seismic data wavefield through a data extrapolation to a depth level below the propagation zone of the surface waves followed by an upward continuation, or extrapolation of the data, back up to the seismic data recording surface. Other current solutions attenuate noise by upwardly continuing the seismic image wavefield through a data extrapolation to a depth level above the highest elevation followed by a downward continuation, or extrapolation of the data, back to the recording surface. Both approaches can be at least marginally effective (albeit inefficient) because the seismic imaging surface waves often manifest themselves as evanescent waves which exhibit exponential decay with distance and are therefore capable of being filtered out during the wavefield extrapolation.